Carrier aggregation was introduced in 3rd Generation Partnership Project (3GPP) Long Term Evolution Advanced (LTE) Release 10 (LTE Rel-10) as a LTE-Advanced feature. Using carrier aggregation, multiple “Component Carriers” (CCs) can be aggregated and jointly used for transmission to or from a single wireless device. Each component carrier can have any of the LTE Release 8 (LTE Rel-8) bandwidths: 1.4, 3, 5, 10, or 20 Megahertz (MHz). Up to five component carriers can be aggregated to give a maximum aggregated bandwidth of 100 MHz. Further, each component carrier uses the LTE Rel-8 structure to provide backward compatibility (i.e., each component carrier appears as an LTE Rel-8 carrier).
FIG. 1 illustrates one example of carrier aggregation. In this example, cells 10-0 through 10-4 having carrier frequencies F0, F1, F2, F3 and F4 can be aggregated. In this example, the cells 10-0 through 10-4 are transmitted by a single base station 12. With respect to a particular wireless device, one of the cells 10-0 through 10-4 serves as a Primary Cell (pCell) of the wireless device, where the pCell handles Radio Resource Control (RRC) connection. The component carrier of the pCell is referred to as the Primary Component Carrier (PCC). Other cells aggregated with the pCell for the wireless device are referred to as Secondary Cells (sCells) having corresponding Secondary Component Carriers (SCCs). All of the aggregated cells for the wireless device are referred to as serving cells of the wireless device.
The coverage areas of the cells 10-0 through 10-4 may differ either due to different component carrier frequencies or due to power planning on the different component carriers. In the example of FIG. 1, the cell 10-0 has the largest coverage area and serves as the pCell for wireless devices A, B, C, D, and F located in the cell 10-0. The cells 10-1 through 10-4 have successively smaller coverage areas and serve as sCells for wireless devices B through F. In this example, wireless device A has no sCell coverage, wireless device B has sCell coverage for one sCell (namely cell 10-1), wireless device C has sCell coverage for two sCells (namely cells 10-1 and 10-2), wireless device D has sCell coverage for three sCells (namely cells 10-1, 10-2, and 10-3), and wireless device F has sCell coverage for four sCells (namely cells 10-1, 10-2, 10-3, and 10-4). Therefore, depending on the position of a wireless device within the pCell, the wireless device may have no sCell coverage or have coverage of one or more sCells.
For a wireless device connected to the pCell on carrier frequency F0 (e.g., wireless device A), the base station 12 normally starts inter-frequency layer 3 (L3) measurements on candidate sCell(s) in order to determine whether the wireless device has any sCell coverage. For instance, the base station 12 normally starts inter-frequency L3 measurements such as, for example, a measurement that triggers an A4 event when the inter-frequency L3 measurement for an sCell becomes better than a threshold. In the LTE specifications, an A4 event occurs when a neighbor cell becomes better than a threshold, which is referred to herein as an A4 threshold. In the example of FIG. 1, with a proper A4 threshold, an A4 event will trigger on carrier frequency F1 for wireless device B to thereby indicate that wireless device B has sCell coverage via the cell 10-1. In contrast, for wireless device F, an A4 event will trigger on carrier frequencies F1, F2, F3, and F4 to thereby indicate that wireless device F has sCell coverage via cells 10-1, 10-2, 10-3, and 10-4. Based on the measurement event triggering, one or more sCells are selected and configured for each wireless device having sCell coverage.
One issue with this normal sCell selection process is that the inter-frequency measurements may require measurement gaps. Measurement gaps are periods during which there is no traffic in both the uplink and downlink directions. Using measurement gaps to perform the inter-frequency measurements for sCell selection will incur 7-15% throughput loss on configured cells depending on the gap pattern configured.
Another issue with the normal sCell selection process is that to perform any measurements (inter-frequency or intra-frequency, gap or gapless measurements) on the candidate sCells, the parameter s-Measure may have to be disabled. As defined in the LTE specifications, when the pCell's Reference Signal Received Power (RSRP) measurement is not below s-Measure, the wireless device is not required to perform any neighbor cell measurements including the measurements on the candidate sCell(s) to save battery. Thus, as illustrated in FIG. 2, the wireless devices A, B, C, D, and F may not perform neighbor cell measurements unless they are outside the indicated s-Measure line, which corresponds to the s-Measure parameter. Thus, in order to guarantee that the measurements on the candidate sCell(s) are being performed by the wireless devices A, B, C, D, and F when using the normal sCell selection process, the s-Measure parameter will have to be disabled which will cause increased wireless device battery consumption.
In light of the discussion above, there is a need for systems and methods for improved sCell selection.